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Crystal structures and dynamic rearrangements of one-dimensional coordination polymers with 4,4′-dipyridylsulfide (dps) have been studied. Reaction of Ni(NO3)2·6H2O with dps in EtOH yielded [Ni(dps)2(NO3)2] ·EtOH (1), which had channels filled with guest EtOH molecules among the four Ni(dps)2 chains. This coordination polymer reversibly transformed the channel structure responding to temperature variations. Immersion of 1 in m-xylene released guest EtOH molecules to yield a guest-free coordination polymer [Ni(dps)2(NO3)2] (2a), which was also obtained by treatment of Ni(NO3)2·6H2O with dps in MeOH. On the other hand, removal of the guest molecules from 1 upon heating at 130 °C under reduced pressure produced a guest-free coordination polymer [Ni(dps)2(NO3)2] (2b). Although the 2a and 2b guest-free coordination polymers have the same formula, they showed differences in the assembled structures of the one-dimensional chains. Exposure of 2b to EtOH vapor reproduced 1, while 2a did not convert to 1 in a similar reaction. Reaction of Ni(NO3)2·6H2O with dps in acetone provided [Ni(dps)(NO3)2(H2O)] ·Me2CO (4) with no channel structure. When MeOH or acetone was used as a reaction solvent, the [Ni(dps)2(NO3)2] · (guest molecule) type coordination polymer, which was observed in 1, was not formed. Nevertheless, the reaction of Ni(NO3)2·6H2O with dps in MeOH/acetone mixed solution produced [Ni(dps)2(NO3)2]·0.5(MeOH·acetone) (5), which has an isostructural Ni-dps framework to 1.

Incorporation of dynamic mechanisms into the channel frameworks have attracted intense attention for the development of new functional materials [1–19]. For example, chemical modifications of the frameworks of zeolites have yielded unique functions such as controlled release of the including guest molecules from channels [19]. These functions are important not only for the development of drug delivery systems, but also for highly effective storage, including of guest molecules. On the other hand, many studies have reported that coordination polymers, which are also called metal-organic frameworks (MOFs), with channel structures, afford a variety of infinite network structures [20]. These compounds have been synthesized from metal sources and organic bridging ligands by a self-assembly process. These coordination materials have been considered as a new class of porous materials because they have often shown unique functions, which were not observed in inorganic materials such as zeolites. For example, heterogeneous catalysis [21–24], high gas storages [25–28], and high selective molecular adsorption [1,13,29–32] have been reported. Many porous coordination polymers cannot retain their channel frameworks after the removal of included guest molecules that were incorporated in the channels when they were prepared. In spite of their fragility, some porous coordination polymers have unique adsorption properties, and can selectively re-include organic guest molecules; and reproduce the initial porous framework.

For years we have focused on coordination polymers that change their structures responding to external stimuli such as temperature variation [6] and present organic solvents [33,34]. As a unique example, we reported a new Ni coordination polymer with 4,4′-dipyridylsulfide (dps) in our previous communication [6]. This compound created unique channels, which changed the channel windows responding to temperature variation. The channels below the critical temperature mechanically captured guest EtOH molecules, and then released them above the temperature. This coordination polymer was comprised of one-dimensional frameworks formulated as [Ni(dps)2(NO3)2], which is designated as “(Ni-dps2) chain” (Scheme 1). This paper describes the unique rearrangement properties of the (Ni-dps2) chains responding to external stimuli such as temperature variations, and the guest release and re-inclusion.

2. Results and Discussion2.1. Overview of the Structural Rearrangement of the Ni-dps System

Scheme 2 summarizes the structures and rearrangement of the (Ni-dps2) chains in Ni-dps compounds. The views are illustrated along the one-dimensional chain direction except for 4. Reaction of Ni(NO3)2·6H2O with dps in EtOH or MeOH produced coordination polymers 1 and 2a, which were constructed by stacks of (Ni-dps2) chains. 1 had two structural phases that reversibly transformed depending on the temperature about -12 °C. The two structural phases observed above and below the critical temperature were designated as 1α and 1β Immersion of a solid sample of 1 into m-xylene released guest EtOH molecules, and converted 1 to 2a. On the other hand, removal of the guest EtOH molecules from single crystals of 1 on heating at 130 °C under reduced pressure produced dried compound 2b as a crystalline solid. Although the data quality was poor due to the cracks, 2b was useful for single crystal X-ray analysis. This means that the guest removal reaction proceeded by the single-crystal-to-single-crystal process [11]. The dried compound 2b reproduced 1 by exposure to EtOH. Although 2a and 2b are guest-free coordination polymers with the same formula, their stacking patterns of (Ni-dps2) chains are different, meaning that 2b is an allotrope of 2a. While 2b converted to 1 reversibly, 2a did not convert to 1 in a similar reaction condition.

The reactions of Ni(NO3)2·6H2O with dps in Me2CO produced coordination polymer 4, which was not constructed by (Ni-dps2) chains, but {Ni(dps)(NO3)2(H2O)}n chains. On the other hand, when the reaction was carried out in MeOH/acetone mixed media, coordination polymer 4, which had an assembled structure like 1α, was obtained.

2.2. Crystal Structures of 1α and 1β

1 was easily obtained as light-blue crystals by diffusion of dps into the Ni(NO3)2·6H2O in an ethanol solution [6]. Figure 1 and 2 compare the crystal structures of 1α and 1β1α which is in the structure phase of 1 at room temperature, crystallizes in the centric space group Ccc2. The structural determination was carried out at 23 °C. The nickel center is based on a distorted octahedron with four pyridine nitrogen atoms and two oxygen atoms from nitrate anions, in which the nitrate anions occupy the axial positions (Figure 1). Each nickel center is bridged by two dps ligands to yield one-dimensional chains with small rhombus cavities (ca. 5 × 5 Å) surrounded by two nickel atoms and two dps ligands.

These chains run along the c axis. There are two crystallographically equivalent chains with different inclinations to the a and b axes each, whose tilting angles of NO3—Ni—NO3 vectors to the a axis are about 15° and −15°. These chains alternatively stack along the a axis, with the nitrate anions being located above and below the square cavities of the adjacent chains. Among four one-dimensional chains, one-dimensional channels with a compressed octahedral shape (ca. 5 × 5 Å) are created along the c axis. Although elemental analysis and structural characterization at lower temperature showed that 1 contained one ethanol molecule per nickel atom, the expected electron densities were not observed in the channels of 1α, despite that we carried out X-ray measurements using several different single crystals. As a result, no atoms could be located in the channels of the X-ray refinement models for 1α. Thus, we concluded that remarkable disorder must exist for the ethanol molecules in the channels at this temperature.

The crystal structure of the second phase, 1β, which forms below the critical temperature, was determined by X-ray analysis at −40 °C by using the single crystal (1α) that was prepared at room temperature. The space group Ccc2 for 1α was changed to the acentric space group Pnc2 for 1β. In contrast to 1α, 1β contained two crystallographically independent nickel centers, which yielded two types of one-dimensional chains that are made of equivalent nickel centers. The two chains are labeled Chain-A and Chain-B in Figure 2d. The inclinations of the two chains to the a and b axes are quite different to those of 1α; the tilting angles of the NO3—Ni—NO3 vectors to the a axis are about 35° for Chain-A and 0° (nearly parallel) for Chain-B. The phase transition accompanies a slide of the Chain-B (or Chain-A) of about 1 Å along the c axis. As a result, the coordinating nitrate anions are off-center above and below the square cavities of the two adjacent chains. The guest EtOH molecules, which were not structurally defined in 1α, were clearly observed in the channel-like cavities of 1β The oxygen atom of the EtOH formed a weak hydrogen bond with an oxygen atom of a coordinating nitrate anion (O(4)—O(7) = 3.096; (2) Å).

The most significant effects of this phase transition on the porous structures are established by the rotation of the coordinating nitrate anions. When the angle of the NO3 plane of the coordinating anion to the channel direction, which is parallel to the c axis, is defined as Φ (Scheme 3), the angles of nitrate anions in 1α are about 45° (and −45°). However, the Φ of nitrate anions in 1β is about 80° (and −80°) for Chain-A, and 15° (and −15°) for Chain-B, respectively. That is, the planes of the nitrate anions of Chain-A are nearly perpendicular to the channel direction. The rotations jutted the nitrate anions into the channels, which resulted in the change of channel shape from “compressed hexagon” in 1α to “T-shape” (5 × 2 + 2 × 3 Å) in 1β. The structural transformation narrowed the channel width from about 5 to 2 Å for the lower half of the channel window. This second phase with diminished channels is regarded as the closed porous phase induced by the temperature switch. In the previous communication, we showed that the including EtOH molecules were securely captured in the closed channels [6].

Weak electrostatic interactions are observed between the two nitrate anions in the adjacent chains; that is, oxygen atom (O(2)) of nitrate in Chain-B electrostatically interacts to nitrogen atom (N(6)) of nitrate in Chain-A. This result indicates that the rotations of nitrate anions are induced by the following mechanism (Scheme 4): the slide of half of the chains at the initial step makes two nitrate anions in the adjacent chains closer. The nitrate anions rotate to induce electrostatic interaction between N and O atoms of nitrate anions in the adjacent chains. As a result, nitrate anions in Chain B protrude into the channel-like cavities.

2.3. Crystal Structures of 2a and 3

The single crystal X-ray analysis data of satisfactory quality was not obtained for 2b, despite several attempts of measurements due to cracking of the crystals occurring on heating. On the other hand, we have found that 2b was isostrucural to [Co(dps)2(NO3)2] (3), which was prepared by treatment of Co(NO3)2·6H2O with dps in EtOH. Since the quality of the single crystal X-ray structure of 3 was better than 2b, we mention the structure of 3 to explain that of 2b here.

The coordination circumstances of 2a and 3 (Figures 3a and 4a) were similar to that of 1α. Although 2a and 3 were both guest-free coordination polymers formulated as [M(dps)2(NO3)2] (M = Ni, Co) constructed by (Ni-dps2) chains and [Co(dps)2(NO3)2] (Co-dps2) chains, their stacking patterns were not same (Figures 3 and 4). Their (Ni/Co-dps2) chains run along the a axis, and stack along the b axis. The Φ angles are about 52° for nitrate anions with N(5) atom and −74° for N(6) in 2b, and the corresponding Φ angles are about 40° for nitrate anions with N(5) and −70° for N(6) in 3. The inclinations of the chains to the b axis are smaller for 2a compared to 3; the tilting angles of the NO3—Ni—NO3 vectors to the b axis are about 6° for 2a and 18° for 3.

2.4. Rearrangement of (Ni-dps2) Chains by Guest Releases and Re-Inclusions

It is usually difficult to retain the structures of the flexible channel frameworks in the absence of guest molecules in the channels. Particularly, channels created among one-dimensional chains could be less stable because the frameworks are not supported three-dimensionally. Nevertheless, the dried compounds often adsorb the guest molecules and re-construct the initial structure. To understand the properties of the host frameworks of 1, we characterized the release and re-inclusion properties of compound 1.

Figure 5 shows the changes of X-ray powder diffraction (XRPD) pattern of 1 responding to removals and re-inclusions of guest EtOH molecules. The XRPD pattern of 1 (Figure 5a) changed to a new one (Figure 5b) when it was dried on heating under reduced pressure. The XRPD pattern of the dried sample is consistent with that of the simulated XRPD pattern for 3 (Figure 5e). When the obtained dried sample was exposed to EtOH vapor for three days, the XRPD pattern of the initial powder was recovered (Figure 5c). This result clearly shows that the dried compound 2b re-produced 1 by contact with EtOH vapor.

We reported that 1β did not release EtOH molecules while 1α released EtOH moleules in m-xylene [6]. The XRPD peaks of the powder sample obtained after the release of EtOH in m-xylene was rather consistent with that of 2a than that of 3, which is isostrucutral to 2b (Supporting Information 1). This result means that 1 converted to 2a by releasing guest EtOH molecules in m-xylene. On the other hand, exposure of EtOH vapor to 2a did not produce 1 as studied by XRPD measurement (Supporting Information 2). These results reveal that the guest adsorption properties are not same between 2a and 2b.

2.5. Thermal Property of Ni-dps Compounds

Reaction of Ni(NO3)2·6H2O with dps in MeOH or acetone did not produce [Ni(dps)2(NO3)2] ·G (G = guest molecules) type coordination polymer, but yielded 2a and 4. 4 does not have (Ni-dps2) chains, but shows one-dimensional coordination framework constructed by connection of Ni(II) centers by dps ligand. Interestingly, we found that the reaction in the mixed solution of MeOH/acetone (1:1) produced 5, which is isostructural to 1α. The crystal structures of 4 and 5 are shown in Supporting Information. Although the positions of guest molecules in the channels were not determined due to the remarkable disorders, the result of elemental analysis implies the inclusions of MeOH and acetone (1:1) guest molecules per two Ni atoms.

While Differential scanning calorimeter (DSC) measurement revealed that 5 showed phase transition similar to 1 (Figure 6), the critical temperature (about −50 °C) is remarkably lower than that of 1. In contrast to 1 and 5, guest-free coordination polymers 2a and 2b did not show phase transition between −100 °C and 20 °C. This result means that the phase transition property is necessary for 1 type porous structure. Moreover, this result shows that kinds of guest molecules largely affect the critical temperature.

3. Experimental Section3.1. Reagents and Materials

All reagents and solvents were purchased from commercial sources and were used as received. The thermal behavior was measured on Shimadzu DSC-60 differential scanning calorimeter (DSC) at a heating rate of 10 °C/min. Elemental analysis was performed on an analyzer Euro Vector EA 3000.

Each single crystal for X-ray analysis measurement was fixed on top of a glass fiber by epoxy glue (1β, 2a, 2b, 3), or sealed in a glass capillary with mother liquor (1α, 4, 5). The data for all structures were measured on a Rigaku Mercury CCD system (MoKα radiation λ = 0.71073 Å). An empirical absorption correction was applied. The structures were solved by the direct method. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms binding to carbon atoms were located on calculated positions, and were not refined but included. The crystallographic data of the compounds in this work is summarized in Table 1. Crystal structures of 1α and 1β, which were reported in previous communication [6], were re-refined in this work to improve their analysis qualities.

4. Conclusions

Unique rearrangements of (Ni-dps2) chains of Ni-dps compounds have been studied. 1 showed two structural phases depending on the temperature. This compound mechanically opens and closes the channels. This dynamic structural change was caused by rotations of nitrate anions, which were induced by the slides of chains. 1 released guest EtOH molecules to yield 2a when immersed in m-xylene, and to yield 2b when heated at 130 °C under reduced pressure. While 2a did not reproduce 1, 2b reproduced 1 by contact with EtOH vapor. The reaction of Ni(NO3)2·6H2O and dps did not produce porous frameworks with (Ni-dps2) chains in MeOH or acetone. Nevertheless, when the reaction was carried out in MeOH/acetone mixed solution, the dynamic porous framework isostructural to 1 was obtained. The existence of channel structures is necessary for the phase transition property responding to temperature variation in the Ni-dps system, and the critical temperature is largely affected by the including guest molecules. The further studies of the dynamic frameworks are in progress.

Supporting Information

Figure S1

XRPD pattern after immersion of powder of 1α in m-xylene for a few days (a) at room temperature. Simulation patterns of 2a (b) and 3 (c). The XRPD pattern of (a) is not entirely consistent with that of (b) because of the effects of crystal morphology and the structural defects occurred during the structural transformation.

Figure S2

The XRPD pattern of powder sample of 2a (a). The XRPD pattern of the powder sample of 2a was exposed to EtOH vapor for three days (b). Simulation pattern of 2a (c). The XRPD patterns between (a) and (b) are not entirely consistent because of the structural defects occurred during the structural transformation.

Figure S3

Crystal structure of 4. The coordination circumstance (a) and stacking pattern in the ab plane (b).

This work was supported by Ogasawara Foundation for the Promotion of Science & Engineering. The authors thank R. Ikeya and K. Terasaki of the Center for Instrumental Analysis for support in obtaining the X-ray diffraction data and elemental analysis data.

Coordination circumstances of 1α (a) and 1β (b). Hydrogen atoms are omitted for clarity. 1β contains two crystallographically independent (Ni-dps2) chains. The nitrate anions in the different chains are connected by electrostatic interactions as shown by dashed line (b).

Figure 2

Crystal structures of 1α (a-c) and 1β (d-f). Ethanol molecules in the channels of 1β are omitted for clarity. Stacking structures of (Ni-dps2) chains along the b axis (a, c, d, f) and c axis (b, e) are exhibited. The channel formed by surrounding four chains is indicated by the rectangles in (b) and (e). Their channel structures with van der Waals radii are revealed in (c) and (f). Except for (c) and (f), the hydrogen atoms are omitted for clarity.

Figure 3

Coordination circumstance of Ni center of 2a (a). Views of stacking aspect of (Ni-dps2) chains of 2a in the ab plane (b), and assembled pattern of the chains along the c axis (c). Hydrogen atoms are omitted for clarity.

Figure 4

Coordination circumstance of Co center of 3 (a). Views of stacking aspect of (Co-dps2) chains of 3 in the ab plane (b), and assembled pattern of the chains along the c axis (c). Hydrogen atoms are omitted for clarity.

Figure 5

X-ray powder diffraction (XRPD) patterns of solid sample of 1 (a), its dried sample obtained on heating at 130 °C under reduced pressure (b), and the powder obtained by exposure of EtOH vapor to the dried sample (c) for three days. The simulation patterns based on the crystal structural analysis of 1α (d) and 3 (e).

Structures and rearrangement aspects of (Ni-dps2) chains for the Ni-dps compounds. The structures are drawn along the chains except for 4.

Scheme 3

Definition of the Φ angle in the (Ni-dps2) chain.

Scheme 4

Plausible nitrate rotation mechanism. The slide of half of the chains make two nitrate anions in the adjacent chains closer, inducing rotations of nitrate anions by the N···O electrostatic interaction between the anions. As a result, half of the nitrate anions jut into the channels.